Gases are compressed for a wide range of applications. For example, compressed gases may be used to store energy, run tools or other pneumatic equipment, provide compact storage of gases, provide conditions to promote chemical reactions and the like. Refrigeration systems and heat pumps also typically include compressors for compressing gases. As air (or any other gas) is compressed, work is being done on the gas. Conservation of energy dictates the energy from the work cannot be lost. In adiabatic compression (adiabatic means there is no heat flow in or out of the system) a significant proportion of the energy from the work done to compress the gas goes into increasing the gas temperature. The end result is hot, compressed gas. Most current technologies for gas compression perform compression that is adiabatic or nearly so.
Many gases behave to a good approximation as ideal gases which obey the ideal gas law:PV=nRT  (1)where P is pressure, V is volume, n is the number of molecules of gas, R is a constant and T is the temperature. When a gas is compressed under adiabatic conditions (no heat flows into or out of the gas during compression) the entropy of the gas remains constant. Therefore, for an ideal gas under adiabatic compression PVγ is constant, where γ is the heat capacity ratio for the gas and so, for an ideal gas, T∝1/V(γ−1), γ generally has a value in excess of 1 so that a decrease in volume, as occurs when a gas is compressed, results in a corresponding increase in the gas temperature. For dry air, γ has a value of about 1.4.
The heating which results from adiabatic compression can lead to inefficiencies because hot compressed gas typically loses heat to its environment. Where a gas is compressed adiabatically, allowed to cool to ambient temperature and subsequently allowed to expand to do work the amount of energy taken to compress the gas is typically about twice the amount of work done. Consequently the overall efficiency of such a round trip compression expansion is only about 50%.
Various attempts have been made to provide compressors that operate on an isothermal cycle. In isothermal compression, the gas being compressed is cooled as it is compressed so that the temperature of the gas remains essentially constant. Such systems have not been widely adopted.